1. Field of the Invention
The present invention relates to a method and apparatus for facilitating the total hip arthroplasty procedure in which both an acetabular cup prosthesis and a femoral implant prosthesis are installed or in a partial hip arthroplasty where one of the components of an earlier arthroplasty are replaced and particularly, to a method and apparatus for assuring that the resulting leg length of the patient is optimized.
2. Description of the Prior Art
Total and replacement arthroplasty procedures employing artificial acetabular cup prostheses and femoral implants have been done for a great many years. The installation of a replacement hip joint prosthesis involves surgically exposing and dislocating the joint, cutting away the head and neck and a portion of the greater trochanter in a femoral neck resection, and reaming the femoral canal to accept the metallic stem of the femoral implant. Femoral implants are available in a variety of lengths and cross-section to fit the shape of the reamed-out femoral canal and are also provided with an angularly disposed femoral neck and spherical head that extend at an angle to the stem to orient the replacement head in the acetabular cup prosthesis. The acetabular cup prosthesis is itself installed in the patient's reamed-out acetabulum. After both components are installed, the spherical head is inserted into the cup and the muscles and tendons that were separated or severed in the surgical exposure are reattached to hold the leg in place.
Numerous articles have been published describing and illustrating total hip arthroplasty procedures employing a wide variety of total hip joint prostheses. For example, the publication entitled "Total Hip Arthroplasty Using a Universal Joint Device," by Raymond G. Tronzo, M.D., published in 1970 by Richards Manufacturing Company, 1450 Brooks Road, Memphis, Tenn. 38116, describes such a procedure employing the products of the Richards Manufacturing Company. A further brochure entitled "PERFECTA Femoral Prostheses With T MATRIX Acetabular Options Surgical Protocol," published in 1990 by Orthomet, Inc., 6301 Cecilia Circle, Minneapolis, Minn. 55439, describes and illustrates the surgical procedure employed in installing Orthomet products. Other manufacturers publish similar instruction manuals or protocols for their products.
A common problem that arises in completing the total hip arthroplasty procedure involves the selection of the appropriate length neck of the femoral implant so that after the procedure is completed, the patient enjoys a normal or enhanced leg length. If only one of the patient's legs is operated on, it is naturally desirable to ensure that the patient's legs are of approximately the same length after recovery from the surgery. Often, due to deterioration in the hip joint, the leg may initially be shorter than desired and the replacement surgery should include restoration of nearly equal leg length. When both hip joints are placed, it may be desirable to equalize and lengthen both legs. In any case, a common problem that is encountered involves the failure of the prosthesis to restore the desired leg length, causing the patient to limp and contributing to a low back pain and aseptic loosening of the cement used to fix the components of the hip joint, as reported by K. B. Turula, M.D., et. al., in "Leg Length Inequality After Total Hip Arthroplasty," Clinical Orthopaedics and Related Research, 1986; 202:163 168.
In this regard, published reports in the literature indicate that the precise desired length of the leg is obtained perhaps no more than about one-fourth of the time. Approximately one-half of the time, the leg can vary from a desired length by up to one half inch. The remaining one-fourth of the time, the leg is too short or too long by substantially greater lengths ranging up to an inch or two. Although patients can tolerate length differences of 1/4-inch or thereabouts, differences in length of 1/2 inch or more are immediately detected by patients and often times are intolerable. When this occurs, it is necessary to reoperate and change the length or provide the patients with heel lifts to equalize leg length and restore proper gait.
The differences in leg length arise from the difference in sizes of the femoral implants, the depth of insertion of the acetabular cup, and the length of the reducted femoral neck. To change the length of a leg, the surgeon simply changes the length of the femoral neck that is used. Once the acetabular cup is in place, it is rarely changed.
A variety of methods have been used to estimate the length of the leg upon implanting a new femoral implant and acetabular cup. Most of the time, a surgeon will attempt to duplicate it with the femoral implant, the size not varying greatly from the size of the proximal femoral portion that is removed.
Some methods have involved measuring the distance from the palpable iliac crest near the waist line to the greater trochanter on the outside of the proximal femur, both of these markers are quite distant from the true hip joint and only indirectly attempt to measure length.
Other methods described in the literature measure the distance from an anchor installed percutaneously above the superior acetabulum to a marker on the exposed greater trochanter. See, for example, S. T. Woolson, M.D., et. al., "A Method of Interoperative Limb Length Measurement in Total Hip Arthroplasty," Clinical Orthopaedics and Related Research, 1985, 194:207-210; W. H. Harris, M.D., "Revision Surgery for Failed Nonseptic Total Hip Arthroplasty," Clinical Orthopaedics and Related Research, 1975, 106:19-26; and N. M. J. McGee, F.R.C.S., et. al., "A Single Method of Obtaining Equal Leg Length in Total Hip Arthroplasty," Clinical Orthopaedics and Related Research, 1985, 194:269-270. In an article by W. E. Knight, M.D. ("Accurate Determination of Leg Lengths During Total Hip Replacement," Clinical Orthopaedics and Related Research, 1977, 123:27-28), a tool is described for measuring the distance between bone screws placed in the exposed ilium about two inches above the margin of the acetabulum and in the greater trochanter of the femur in line with the iliac screw in the coronal plane. The tool is positioned laterally to the pins and the measurement is made parallel to the femur, which is not necessarily parallel to the weight-bearing axis.
These methods are flawed as they measure two dimensions, length (vertical dimension) plus lateralization (horizontal translation of the femur). The hip and leg position must be virtually identical when pre-op and post-op measurements are made to insure predictable results. In practice, the horizontal offset and leg position are difficult to replicate. A better surgical procedure for obtaining exact leg length is much to be desired.